This disclosure relates generally to orifice fittings for measuring fluid flow rates through pipes or other conduits. More particularly, the disclosure relates to a pressure equalization system for use in orifice fittings.
In pipeline operations and other industrial applications, flow meters are used to measure the volumetric flow rate of a gaseous or liquid flow stream moving through a piping section. Flow meters are available in many different forms. One common flow meter is an orifice meter, which includes an orifice fitting connected to the piping section. The orifice fitting serves to orient and support an orifice plate that extends across the piping section perpendicular to the direction of the flow stream. The orifice plate is generally a thin plate that includes a circular opening, or orifice, that is typically positioned concentric with the flow stream.
In operation, when the flow stream moving through the piping section reaches the orifice plate, the flow is forced through the orifice, thereby constricting the cross-sectional flow area of the flow. Due to the principles of continuity and conservation of energy, the velocity of the flow increases as the stream moves through the orifice. This velocity increase creates a pressure differential across the orifice plate. The measured differential pressure across the orifice plate can be used to calculate the volumetric flow rate of the flow stream moving through the piping section.
A dual chamber orifice fitting embodies a special design that enables the orifice plate to be removed from the fitting without interrupting the flow stream moving through the piping section. This specially designed fitting has been known in the art for many years. U.S. Pat. No. 1,996,192 was issued in 1934 and describes an early dual chamber orifice fitting. Fittings with substantially the same design are still in use in many industrial applications today. Although the design has remained substantially unchanged, operating conditions continue to expand and dual chamber fittings are now available for a wide range of piping sizes and working pressures.
A cross-sectional view of common dual chamber orifice fitting 12 is illustrated in FIG. 1. Orifice fitting 12 includes body 16 and top 18. Body 16 encloses lower chamber 20, which is in fluid communication with the bore 34 of a pipeline. Top 18 encloses upper chamber 22 and is connected to body 16 by bolts 17. Aperture 30 defines an opening connecting upper chamber 22 to lower chamber 20. Valve seat 24 is connected to top 18 by bolts 28 and provides a sealing engagement with slide valve plate 56, which is slidably actuated by rotating gear shaft 54. Lower drive 36 and upper drive 38 operate to move orifice plate carrier 32 vertically within bore 34 and fitting 12 between lower chamber 20 and upper chamber 22. Orifice plate carrier 32 can be removed from fitting 12 through upper chamber 22 by loosening bolts 46, which engage locking bar 44 to compress sealing bar 40 and sealing gasket 42 against top 18. Orifice plate carrier is thus selectably disposable between the fully seated position in bore 34 and the upper portions of fitting 12.
In operation, as shown in FIG. 1, aperture 30 is closed by slide valve plate 56 hydraulically isolating upper chamber 22 and lower chamber 20. Pressurized fluid flow in bore 34 passes through orifice 52, which is located on an orifice plate 50 supported by orifice plate carrier 32 that sealingly engages the wall of bore 34. Pressure up and downstream of orifice plate 50 is measured via meter tap holes or communication ports 66. The measured pressure differential across orifice plate 50 is then used to estimate the rate of fluid flow through fitting 12. In order to obtain accurate estimates of the flow rate through fitting 12, all of the flow moving through the pipeline must pass through orifice 52. If any flow by-passes or flows around orifice 52, an error in the measurement of the pressure differential across orifice plate 50 occurs. To prevent flow from bypassing orifice 52, a seal 64 is placed around orifice plate 50, between plate 52 and carrier 32.
When lower chamber 20 has a lower pressure than bore 34, the pressure in bore 34 will tend to urge orifice plate carrier 32 upward and into lower chamber 20, potentially causing misalignment between orifice 52 and bore 34 that can decrease measurement accuracy. Further, seal 64, which is usually constructed from an elastomer or polymer, may fail due to the pressure differential between bore 34 and lower chamber 20. In order to counter the pressure differential, an equalization flow path or weephole 60 is included between lower chamber 20 and bore 34. Weephole 60 provides fluid communication between bore 34 and lower chamber 20, and thus, allows pressure to equalize across orifice plate carrier 32. Weephole 60 is located upstream of orifice 52 so as to be located in the region of highest pressure within bore 34.
In some applications, such as metering for bulk storage facilities, it may be desirable to be able to operate an orifice fitting with flow in either direction through the fitting, in order to measure the alternating flow into and out of the facility. However, if weephole 60 is positioned downstream from orifice 52, the pressure in bore 34 proximate weephole 60 will be less than the pressure in bore 34 that acts on orifice plate carrier 32, thereby creating a pressure differential across carrier 32 and urging carrier 32 into lower chamber 20. Further, seal 64 may tend to expand radially off of orifice plate 50. Once seal 64 is compromised in this manner, pressure differential measurement accuracy is lost.
To enable the measurement of fluid passing through a fitting in either direction, the weephole may be sealed by welding, and a bypass system coupled to the fitting. In some configurations, the bypass system, which replaces the weephole, includes two tubes. One tube is coupled between a meter tap hole to one side of the orifice plate and the lower chamber, while the other tube is coupled between a meter tap hole to the other side of the orifice plate and the lower chamber. A valve is positioned along each tube to permit or prevent fluid flow therethrough.
In operation, the valve positioned along the tube coupled to the upstream meter tap hole is open, while the other valve is closed. Some pressurized fluid passes from the bore through the upstream tube into the lower chamber to provide pressure equalization between the lower chamber and the bore of the fitting upstream of the orifice plate. Thus, the upstream tube of the bypass system performs the same function as a weephole in a uni-directional fitting. Pressure differential measurements may be taken across the orifice plate, as described above.
When the direction of flow through the bi-directional fitting is reversed, the position of each valve is also reversed. What was previously the upstream valve, now the downstream valve, is closed. Similarly, what was previously the downstream valve, now the upstream valve, is opened. With the valve positions reversed and the upstream tube again performing the function of a weephole, pressure differential measurements may be taken across the orifice plate with flow passing through the fitting but in the opposite direction.
While these types of bypass systems offer a means for converting a fitting from uni-directional to bi-directional, these bypass systems are not without their shortcomings. The individual components of the bypass system are costly and can be difficult to install. Once installed, these systems often leak. Since the bypass system is external to the fitting, the tubing and valves are vulnerable to surrounding conditions. An inadvertent impact to the tubing and/or valves, e.g., during transport of the fitting, may cause damage to the bypass system.
Therefore, there remains a need in the art for a bi-directional dual chamber orifice fitting that provides pressure equalization across the orifice plate carrier while overcoming these and certain other limitations of the prior art.